In centrifugal fans, a scroll casing is formed with a bell-mouth shaped air admitting portion in its inlet-port face, which is a face having an opening, an impeller is accommodated in the scroll casing, and air flow is generated by the rotation of the impeller. In such a centrifugal fan, a noise generated in the scroll casing due to the rotation of the impeller or the operation of a drive motor is emitted through the air admitting portion.
In Patent Literature 1, a structure is disclosed in which an orifice that corresponds to the air admitting portion protrudes on an inlet-air-flow upstream side of the lower end of an inlet casing having an inlet port. In the invention disclosed in Patent Literature 1, the inlet casing corresponds to the scroll casing and the lower end of the inlet casing having the inlet port corresponds to the inlet-port face having the opening.